Up to the present time it has been proposed to use either germanium or gallium-arsenide as the substrate for solar cells in which the principal active junction is formed of n-type and p-type gallium-arsenide. Substrates of gallium-arsenide have been preferred for their electrical properties in view of the problems encountered with germanium substrates. These problems have in part involved the "cascade effect", in which some of the total output arises from the junction of gallium-arsenide with the substrate, which is particularly responsive to infrared energy, and which has a relatively high temperature coefficient.
These problems, and the standard methods for making solar cells are discussed in the following articles:
1. MOCVD AlGaAs/GaAs Solar Cells, by M. Kato et al.,; Copyright 1985 IEEE; Publication No. 0160-8371/85/0000-0014; PA0 2. High Efficiency GaAs/Ge Monolithic Tandem Solar Cells, by S.P. Tobin, et al.; IEEE Electron Device Letters, Vol. 9, No. 5, May, 1988, pp. 256-258; PA0 3. High Volume Production of Rugged High Efficiency GaAs/Ge Solar Cells, by Y.C.M. Yeh, F. Ho, et al.; IEEE Photovoltaic Specialist Conference; Las Vegas, Nevada, September 1988; PA0 4. High Altitude Current-Voltage Measurement of GaAs/Ge Solar Cells, by R.E. Hart, Jr., et al.; IEEE Photovoltaic Specialist Conference; Las Vegas, Nevada, September 1988.
Incidentally, concerning the abbreviations for elements which appear in the titles of the foregoing articles, and which are used to some extent in the present specification, the element "aluminum: is abbreviated to "Al"; the element "germanium" is abbreviated to Ge; the element "arsenic" is abbreviated to As; and the element "gallium" is abbreviated to Ga. The designation of a solar cell "GaAs/Ge", indicates that it is gallium-arsenide solar cell grown on a germanium substrate. As is well known in the semiconductor field, silicon and germanium are Group IV semiconductor elements, with four valence electrons, and may be made n-type with an excess for electrons by "doping" with the addition of a similar element having five valence electrons, or may be made "p-type", with a deficiency of electrons, by doping with a similar element having only three valence electrons. Gallium, a Group III element, and arsenic, a Group V element, on either side of germanium in the periodic table, have often been used for "doping" germanium; and when used together, without germanium, form gallium-arsenide which is a good semiconductor. GaAs and Ge materials have several properties with close similarity, notably lattice constant (5.6434.ANG.vis. 5.65748.ANG.) and thermal coefficients of expansion (5.8 .times.10.sup.-6 /.degree. C. vs 5.8 .times.10.sup.-6 /.degree. C.). These features make crystal growth of one substance on the other favorable.
Now, returning to the background of this invention, germanium would be preferred as a substrate for gallium-arsenide solar cells, for a number of reasons. First, germanium has greater fracture toughness than gallium-arsenide, as a substrate; an 8 mil thick germanium slice is twice as strong as a 12 mil thick gallium-arsenide slice. Cost is another factor favoring germanium as it is 30 to 40% less expensive than gallium-arsenide. Also, germanium wafers at 8 mils thick are 34% lighter in weight than 12 mil thick gallium-arsenide, and weight is an important factor for space applications, for example. However, the adverse effects mentioned above, have inhibited the use of germanium substrates.
Another problem which has been troublesome when germanium has been used, is the "self-doping" which occurs at high temperatures when the germanium substrate is exposed to the gases used to deposit gallium-arsenide. With germanium having a melting point of about 937.degree. C., and vapor deposition of gallium-arsenide occurring at temperatures up to 780.degree. C., some germanium may be volatilized and adversely affect the critical doping of other solar cells being processed within the same enclosed volume during production manufacturing. This effect has in the past been blocked by a special cap of gallium-arsenide, involving a process which is relatively costly.
Accordingly, the principal objects of the present invention are to overcome the problems which have been encountered with forming gallium arsenide devices on germanium substrates and to provide in this case a superior solar cell.